With the growing use of portable electric products such as video cameras, mobile phones, portable computers, and the like, significance of secondary batteries being mainly used as their energy sources are rapidly increasing.
A lithium secondary battery has a high energy density per unit weight and allows quick charging, when compared to other conventional secondary batteries such as a lead storage battery, a nickel-cadmium battery, a nickel-hydrogen battery, and a nickel-zinc battery, and thus, its use is on an upward trend.
As opposed to a disposable primary battery, a secondary battery is rechargeable and is being studied very actively in high-tech fields, for example, digital cameras, cellular phones, laptop computer, power tools, electric bikes, electric vehicles, hybrid vehicles, high-capacity energy storage systems, and the like.
For example, a secondary battery may include a nickel-cadmium battery, a nickel-metal hydride battery, a nickel-hydrogen battery, a lithium secondary battery, and the like. Among them, a lithium secondary battery has an operating voltage higher than or equal to 3.6V, and is used as a power source of portable electronic appliances or high output devices such as electric vehicles, hybrid vehicles, power tools, electric bikes, energy storage systems, and uninterruptible power supplies (UPS) by connecting a plurality of secondary batteries in series. A lithium secondary battery has three times higher operation voltage than that of a nickel-cadmium battery or a nickel-metal hydride battery and an excellent characteristic of energy density per unit weight, and thus, is being increasingly used.
A lithium secondary battery may be classified into a lithium ion battery using a liquid electrolyte and a lithium ion polymer battery using a solid polymer electrolyte, based on a type of an electrolyte. Also, a lithium ion polymer battery may be divided into an all-solid-state lithium ion polymer battery containing no electrolyte liquid and a lithium ion polymer battery using a gel polymer electrolyte containing an electrolyte liquid, based on a type of a solid polymer electrolyte.
A lithium ion battery using a liquid electrolyte is generally used in a shape of a cylindrical or prismatic metal can used for a container that is sealed hermetically by welding. A can-shaped secondary battery using a metal can as a container has a fixed shape, which has limitations on design of an electric product using this as a power source and its volume reduction. Accordingly, a pouch-type secondary battery fabricated by putting a cell assembly and an electrolyte into a pouch casing made from films and forming a seal has been developed and is being used.
The pouch casing is a flexible casing including a metal substrate, and has a structure in which a sealing layer made from a polymer is laminated onto a surface facing the cell assembly to be sealed by heat and pressure. Accordingly, the cell assembly is put in the pouch casing, and heat and pressure is applied along a periphery of the pouch casing to hermetically seal the pouch casing by melting the sealing layer.
Meanwhile, when sealing the pouch casing, heat and pressure is applied along the edges of the pouch casing as close as possible to the cell assembly within an allowable range. In this instance, after a sealing material constituting the sealing layer melts at a portion to which heat and pressure is applied, the sealing material flows in a direction perpendicular to a direction in which the pressure is applied, that is, a direction in which the cell assembly is disposed. Thus, usually, the melted sealing material comes in contact with the cell assembly. In this case, there is an advantage that a location of the cell assembly within the pouch casing can be securely fixed within the pouch casing. That is, the melted sealing material acts as an adhesive to fix the cell assembly. Accordingly, in case the secondary battery is used in a portable terminal such as a mobile phone or a laptop computer, durability of the secondary battery may be further improved using a flow phenomenon of the sealing material.
However, surprisingly, the Inventors of this application discovered the fact that such flow phenomenon of a sealing material is a durability degradation factor of a secondary battery depending on a usage field of the secondary battery.
That is, a sealing material solidified (hereinafter referred to as ‘sealing bridge’) after coming in contact with the cell assembly during sealing of the pouch casing serves as a bridge that physically connects the sealing layer of the pouch casing to the cell assembly. Accordingly, when the cell assembly waggles due to an external impact, stress is applied to an area in which the sealing bridge is formed. This stress causes cracking in the sealing layer near the sealing bridge. The resulting crack exposed through the metal substrate of the pouch casing causes a corrosion reaction of the metal substrate in the presence of the electrolyte solution, thereby resulting in life reduction of the pouch casing and insulation degradation of the pouch casing.
Accordingly, a structure for solving the problem resulting from a sealing bridge needs to be introduced to a pouch-type secondary battery used in an environment in which vibrations and impacts of various strengths are continuously applied thereto, for example, an electric vehicle or a hybrid vehicle.